Lead-free (Pb-free) solder bumping technology is replacing the use of high-Pb alloys for chip package interconnection in advanced integrated circuit (IC) technologies. The increasing demand for smaller solder bumps, higher current per bump, larger die sizes, and other features drive increasingly challenging requirements for bump electrical, thermal, and mechanical performance, particularly with flip chip plastic ball grid array organic laminate packages.
Standard processing methods may encounter challenges, however. Due to the coefficient of thermal expansion of the solder, following chip package join by reflowing and annealing the solder, the solder may undergo stress as it cools. This physical stress may cause breakage of either the IC chip, the solder, or the connection.
“Cladding” of Pb-free solder bumps by encasing the solder bumps with Ni, Cu, or similar alloys protects the solder bumps, and provides a redundant current connection in the event of electromigration degradation within the solder, in the form of excessive intermetallic compound formation, voiding, or even mechanical failure. Typical processing used to form a solder bump with cladding is complex, however, and can be difficult to integrate into manufacturing.